Regular Expressions

A regular expression describes a set of criteria that a string can match. In the validation rules of an entity a regular expression can be used to validate whether an attribute of type String matches these criteria.

A regular expression has the following properties.

Common

Name

The name can be used to refer to the regular expression from a validation rule of an entity.

Documentation

This is for documentation purpose only; it is not visible in the end-user application that you are modeling.

Expression

The expression defines the criteria that a string should be checked against in a formal, internationally standardized regular expression language.

The second, third and fourth characters are digits in the range 0 to 9.

The last two characters are letters, as expressed by the last two subexpression [A-Za-z], which indicate that the last two characters should be in the range A-Z or the range a-z.

Between the digits and the letters there can be a space, as expressed by the subexpression which consists of a space and a question mark. The question mark indicates that the space occurs never or once.

Subexpressions

A regular expression consists of a sequence of subexpressions. A string matches a regular expression if all parts of the string match these subexpressions in the same order.

A regular expression can contain the following types of subexpressions:

[ ] – a bracket expression matches a single character that is indicated within the brackets.
For example:

[abc] matches “a”, “b”, or “c“

[a-z] specifies a range which matches any lowercase letter from “a” to “z“

These forms can be mixed: [abcx-z] matches “a”, “b”, “c”, “x”, “y”, or “z”, and is equivalent to [a-cx-z]

The “-” character is treated as a literal character if it is the last or the first character within the brackets, or if it is escaped with a backslash \

In the zip code example above the four bracket expressions [0-9] indicates that the first 4 characters should be digits, and the two bracket expressions [A-Za-z] indicate that the last two characters should be letters

[^ ] – matches a single character that is not contained within the brackets. For example, [^abc] matches any character other than “a”, “b”, or “c”. [^a-z] matches any single character that is not a lowercase letter from “a” to “z”. As above, literal characters and ranges can be mixed.

{m,n} – matches the preceding element at least m and not more than n times.
For example, a{3,5} matches only “aaa”, “aaaa”, and “aaaaa”.

{n} – matches the preceding element exactly n times. For example, using this construct the expression of the Dutch zip code example above would be:[0-9]{4} ?[a-zA-Z]{2}

. – a dot matches any single character. If you want to match a dot, you can escape it by prefixing it with a \ (backslash).

A literal character is a character that does not have a special meaning in the regular expression language matches itself. Example:

In the Dutch zip code example above the space is such a character that just matches itself.

\w – a word: a letter, digit, or underscore. \w is an abbreviation for [A-Za-z0-9_].
\d – a digit” an abbreviation for [0-9].

Quantifiers

The number of times that a subexpression may occur in a string is indicated by a quantifier after the subexpression. If no quantifier is present, the subexpression must occur exactly once.

The following quantifiers can be used:

Quantifier

Description

?

The preceding subexpression should occur not or once.

*

The preceding subexpression occurs any number of times.

+

The preceding subexpression should occur once or more.

No quantifier means that the preceding subexpression should occur exactly once.